An antenna device of this type is e.g. known from WO91/17533. In supermarkets such devices are positioned in pairs on either side of the passageways between the cash registers and one of the two antennas serves as a transmitting antenna and the other as a receiving antenna. An electromagnetic alternating field emitted by the transmitting antenna can excite to resonate resonant circuits on tags, which are fitted to the products to be protected. The field change resulting from the presence of an excited resonant circuit can be detected by means of the receiving antenna and used for triggering a theft alarm.
Screens or shields are associated with the antennas, so as to produce surroundings which, with respect to the transmitting and receiving antennas, are very clearly defined and independent of the presence of electrically conductive parts in the cash registers. In the case of WO91/17533 they consist of a screening plate or a screening grid. The screens screen the antennas against unwanted signals, which are produced by electronic devices within the cash registers. The screens also ensure that an alarm is not triggered, if a product provided with the resonant circuit is correctly placed on the conventionally used conveyor belt and is moved on the latter on the back e.g. passed the transmitting antenna to the cash register.
However, such screens have a disadvantageous influence on the antenna characteristics. However, this influence decreases with increasing distance between the antenna wire and the screens. Due to the generally limited width of the passages between the cash registers, this distance cannot be made randomly large. WO91/17533 considers that a spacing of 4 cm is adequate. For the mechanical stabilization of the antenna wire in WO91/17533 a tube is provided, but it is also mentioned that the antenna, the screen and the associated transmitting and reception electronics can be integrated into a unit.
An antenna device for detecting magnetic strip tags or labels, with an antenna wire running in the form of several loops and a screening plate integrated into a casing is known from EP-A2-352,513. The casing is in this case longitudinally divided into a front and a rear casing part. The casing is very voluminous. The necessary stability is given to the overall arrangement by carrying and stiffening elements integrated into the casing. All the cavities in the casing are also foam-filled.